John Ehrlichman's five years as a top presidential aide, his voluminous notes , and his obvious writing talent should have ensured that his third book would be extraordinary.

Instead it is disappointing and disturbing, for it focuses almost exclusively on the negative. Recognition for achievement is rarely given. No high government official in Washington during the Nixon years seems beyond the range of the belittling and often gossipy Ehrlichman appraisal. Except one: Ehrlichman himself (though he indulges in a small amount of dignified self-criticism).

All this makes for entertaining reading. But in the end it is so unfair, unbalanced, and mean-spirited that it undoes the author's credibility.

Certainly there is much about Washington officials that is deplorable -- as anyone who has spent much time there knows. Yet many have redeeming qualities and worthy achievements; that is how government progresses. For the author not to note this is a grave flaw.

Ehrlichman's position in the Nixon administration -- and the book's title -- seems to promise a measured view of what really goes on behind the scenes at the power center. But anyone deciding whether to read ''Witness to Power'' needs to realize that is not what he or she will find. Rather, it is a peek at the personal peculiarities of the top players within the Nixon administration. And a derogatory peek -- sometimes bitterly so -- toward practically everyone.

This negative perspective would be understandable, perhaps, from a person who had lost as much as Ehrlichman had -- his high government position, his honor, his freedom (after two court convictions and subsequent imprisonment), and his first wife. But unfortunately, this also was too often the perspective of Ehrlichman before his troubles, as those who knew him in Washington found when he was not only a witness to power but also a wielder of it.

In virtually unremitting succession, Ehrlichman describes all of Congress, broad-brush, as lazy; the two judges who presided over his trials as determined in advance to convict him (he was found guilty of conspiracy and other charges in the break-in of Daniel Ellsberg's psychiatrist's office and in the Watergate coverup); National Security Advisor Henry Kissinger as extremely vain and insecure, persistently trying to backstab the secretary of state; the secretary of defense as devious; most Washington reporters as ''basically lazy or burned-out.''

The most belittled figure in the book is President Nixon. Ehrlichman says the former chief executive spent only half his time on substantive issues, spent ''countless hours of . . . rambling and rumination,'' and so despised confrontations that he pushed assistants to do most of his dirty work. Nixon's achievements are mentioned only in passing.

Among all these items are some newsy assertions. For one, that then-President Nixon, then-Attorney General John Mitchell, and Warren Burger, chief justice of the US Supreme Court, discussed issues before the court. (Mitchell denies the charge.) Another assertion: that the Joint Chiefs of Staff systematically spied on Henry Kissinger.

All of this, Ehrlichman assures us, is taken from the copious notes he kept from '69 to '74, when he occupied two successive high-level White House posts. Fascinating word pictures and deft phrases pull the reader quickly along (''Trice was a tall, bald stork of a politician, and he was perched in one of the more elaborate Capitol Hill nests.''). Yet too much of the wording, like the broader pictures it paints, is gratuitously cutting.

Ehrlichman is hardly the first to discover the yawning gap between the chaste theory of American government taught in eighth-grade civics class, and the who-do-you-know-and-what-do-you-know-about-him governmental style too often practiced. His book could have been distinguished, had it consisted of his inside information on Nixon-era officialdom - especially the interplay among high administration officials -- viewed without self-serving bias and written in the readable Ehrlichman style. It's a pity the opportunity wasn't taken.